[CdkTable] expose _insertRow to derived classes

#### Bug, feature request, or proposal: proposal

#### What is the expected behavior?
`_insertRow` is visible to inheriting classes

#### What is the current behavior?
`_insertRow` is private

#### What is the use-case or motivation for changing an existing behavior?
In `_insertRow` the context is created (`RowContext`).
The row context is very useful as a metadata storage for the row, storing information about the row that helps when we want to create features that require state.

For example, flagging if the row is currently being edited, which cell is edited. If the row is being dragged or not etc...

This is also true for a specific cell, metadata for it can be saved on the row context.

#### Is there anything else we should know?
Because `_insertRow` calls `_renderRow`, which is also private, it will require exposing `_renderRow` as well, which is dont think is such a bad thing.

But, if we want to avoid that, maybe it will be better to expose a new protected method, `_createRowContext(data: T): RowContext ` that people can extend

Beginne damit, die Methoden _insertRow und _renderRow von CdkTable sowie die Erstellung von RowContext zu lesen. Bestimme, ob abgeleitete Klassen auf die vorhandenen Methoden oder auf einen geschützten Einstiegspunkt zur Kontexterstellung zugreifen sollten; erledigt ist dies, wenn die gewählte API Zeilenmetadaten für abgeleitete Klassen unterstützt, ohne das Verhalten mehrdeutig zu machen.

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